High ratio planetary type traction roller transmission
BACKGROUND OF THE INVENTION 
     The invention relates to a high ratio planetary type traction roller transmission utilizing two planetary transmission stages. 
     Such high ratio transmissions are needed in large numbers for converting rotary motion from a high speed motor, for example, for low speed, high torque applications. However, various machines utilize various speeds and torques so that, for different applications, different transmissions are required with different sun rollers, different planetary rollers and different traction rings. 
     The motion transmitting traction surfaces of such transmissions need to be engaged with one another with forces sufficient to transmit the respective torque and, as the torque changes within a transmission from stage to stage, so do the engagement force requirements. One could of course provide for maximum engagement forces for all the traction surfaces but then all the traction surfaces would be exposed to full loads at all times which not only reduces their lives but also reduces transmission efficiency and would require the use of relatively expensive materials for the traction surface areas. 
     In the arrangement according to U.S. Pat. No. 4,060,010, for example, the traction roller engagement forces are the same for all the stages although the torque at the drive shaft end of the transmission is by far lower than at the output shaft end. 
     The same can be said for the arrangement shown in U.S. Pat. No. 3,490,311 in which a single engagement cam structure is provided for both main planetary drive structures providing for both the same engagement forces. 
     It is the principal object of the present invention to provide a two stage planetary type traction roller transmission in which the traction surfaces are subjected essentially only to the forces needed for the transmission of the appropriate torque and wherein, with the same transmission housing and traction ring structures, various transmission ratio planetary and sun rollers may be utilized, thereby providing for easy changeover to different transmission ratios or for a relatively large variety of different ratio transmissions with relatively few parts. 
     SUMMARY OF THE INVENTION 
     In a traction roller transmission which includes in a housing two planetary type transmission structures arranged in series for a high ratio speed change, each of the planetary type transmission structures has spaced concentric sun rollers and traction rings with planetary type traction rollers supported by support members in the space between, and in engagement with, the sun roller and the traction ring. An input shaft carries the sun roller of one of the transmission structures and the traction roller support member of said one transmission structure carries the sun of the other transmission structure while the traction roller support member of the other transmission structure is connected to the transmission output shaft. The traction rollers are supported at predetermined angles with respect to the transmission axis and the traction rings are axially curved and forced toward one another so as to engage therebetween the traction rollers, thereby generating radial traction surface contact forces which depend on the angle of inclination of the traction rollers, which angle is so selected that the traction surface contact forces correspond to the torque to which the traction surfaces of the planetary type transmission structures are subjected. With such an arrangement the traction surfaces are not subjected to unnecessarily large contact forces and structures with various ratios and correspondingly various angles may be utilized in connection with one type or even the same housing and traction rings. 
    
    
     SHORT DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of the transmission according to the invention; and 
     FIG. 2 shows an embodiment with concentric input and output shafts. 
    
    
     DESCRIPTION OF A PREFERRED EMBODIMENT 
     As shown in FIG. 1 the transmission includes a housing 1 which receives the transmission structure and is provided with a cover 2. Coaxial input and output shafts 3 and 4 are rotatably supported in the housing end wall 5 and the cover 2 by antifriction bearings 6 and 7. 
     The input shaft 3 is provided at its inner end with a sun roller structure 8 which forms the center roller of a first planetary traction roller structure 9 having a first traction ring 10 supported in the housing 1 in spaced relationship from the sun roller structure 8 and a first set of planetary type traction rollers 11 disposed in the space between the ring 10 and the sun roller 8 and in engagement with both of them. The traction rollers 11 are rotatably supported on a first planetary roller support structure 12 which is part of an intermediate transmission member 13 supporting the traction rollers 11 at a predetermined angle α with respect to the axis of the input shaft 3. The intermediate transmission member 13 is further provided with a roller structure 14 which forms the sun roller of a second planetary type traction roller transmission 15 having a second traction ring 16 supported with housing 1 in spaced relationship from the sun roller 14 and a second set of planetary type traction rollers 17 disposed in the space between, and in engagement with, the sun roller 14 and the traction ring 16. The traction rollers 17 are rotatably supported on a second planetary roller support structure 18 so as to be disposed at a predetermined angle β with respect to the axis of the output shaft 4. The second planetary roller support structure 15 is mounted on, or part of, the output shaft 4. The second sun roller structure 14 has a slightly conical surface defined by a cone having an apex preferably coinciding with the axes of the traction rollers 17 on the axis of the output shaft 4. The sun roller 8 of the first planetary type transmission structure is shown in FIG. 1 with a cylindrical surface, an arrangement wherein the apex of the cone defined by the conical surface of the traction rollers 11 is disposed on the cylindrical envelope of the sun roller 8. 
     Both traction rings 10 and 16 have inner traction surfaces 19 and 20 which are curved in axial direction such that the planetary traction rollers 11 and 17 engage them properly along a circular line independently of the angular inclination α or β of the planetary traction rollers. The surface areas 19 and 20 are shown in the drawings as being symmetrical but need to extend only from the innermost point only far enough to accommodate the largest angles α or β utilized in connection with a particular transmission design. It is noted however that preferably the rings 10, 16 are provided with symmetrical surface areas as shown in the drawings since this permits their reversal when worn on one side thereof. One of the traction rings, that is, traction ring 10, is mounted so that it is not rotatable whereas the other, that is, ring 16, is rotatably supported but has an axial cam structure 21 associated therewith of the type described, for example, in applicant&#39;s U.S. Pat. No. 4,052,915 which forces the ring 16 axially toward the ring 10 with a force that depends on the torque transmitted thereto by the rollers 17, thereby causing firm engagement of the traction rollers with their respective sun and ring structures. Cam springs 22 extend between the traction rings 10 and 16 in order to insure return of the ring 16 when the reaction torque forces thereon diminish. A spacer ring or a set of rings 23 is disposed between the traction ring 10 and the housing 1, the rings 23 being so selected as to insure a slight preload of all the traction surfaces. 
     The planetary traction rollers 17 are rotatably supported on shaft members 24 which are mounted at one of their ends to the support structure 18 and carry at their opposite ends a connecting ring 25 which interconnects all the shaft members 24 and which is adapted to transmit the axial thrust forces generated by the loading cam structure 21 to the intermediate transmission member 13 by way of an axial thrust bearing 26 arranged therebetween. Axial thrust bearings 27 and 28 are also arranged between the traction rollers 17 and the connecting ring 25 and between the traction rollers 11 and their support structure 12 for the transmission of the axial engagement forces. 
     The input shaft sun structure as shown in FIG. 1 is essentially cylindrical since cylindrical structures are easy to manufacture and since, because of the relatively high speed of the input shaft, the torque and the required surface contact forces are relatively low. Slight spin on its surface is therefore not damaging particularly if the surfaces of the traction rollers 11 are slightly curved as they may be because of the relatively low torque to be transmitted. 
     The slight curvature of the traction rollers and the angles α and β of their inclination are so selected as to provide the appropriate contact forces. A larger angle α, for example, provides for smaller contact forces in the planetary transmission structure 9 relative to those applied in the planetary transmission structure 15 since the axial forces applied to the traction rings are the same for both transmission structures. 
     The value for the relative engagement forces can be selected as desired depending on the selected transmission ratios. Various roller and sun sizes may be used in the same size housing simply by using different traction roller and sun structures. The housing and the traction rings 10 and 16 which are relatively expensive need not be different. They can accommodate any suitable roller size and inclination. 
     As mentioned earlier the arrangement permits to make different ratio transmissions with a single housing and a relatively small number of different internal parts. Depending on the selection of sizes for the rollers and sun structures a plurality of different ratio transmissions can be provided. With, for example, components for four ratios of 4, 6, 9 and 12, depending on their combination, twelve different transmissions with various ratios of 16, 24, 36, 48, 54, 72, 81 and 108 can be provided. 
     FIG. 2 shows an embodiment similar to that of FIG. 1 and therefore the same reference numerals are used where appropriate. However, as shown the input shaft 3&#39; extends concentrically through the output shaft 4&#39; and through a central bore 30 in the intermediate transmission member 13&#39; and carries at its end a sun roller structure 8&#39; which, in this case, is slightly conical for spin-free motion transmitting engagement with the planetary traction rollers 11. The shaft 3&#39; is supported in the output shaft by a bearing 31 which is capable of taking up also the axial component of the engagement forces applied by the inclined planetary rollers 11 to the sun roller structure 8&#39;. 
     As mentioned earlier, the inclination angles α and β for the planetary rollers 11 and 25 are selected such that the engagement forces generated by their engagement with the traction rings 10 and 16 are appropriate to transmit the torques to be transmitted at the various engagement locations. For example, the inclination angle α of the traction rollers 11 is larger than the inclination angle β of the traction rollers 25 since the torque to be transmitted by the rollers 11 is smaller than that to be transmitted by the rollers 25. 
     To determine the appropriate angles a computer program was developed which supplies the transmission design parameters when given the basic design features such as transmission ratio and maximum input or output torque. A printout is supplied as Att. 1 giving the design features of a particular drive as determined by the program. ##SPC1##